Variable Coherence Integration for the Location of Weak Signals

ABSTRACT

In a network-based Wireless Location System (WLS), geographically distributed Location Measurement Units (LMUs) must be able to detect and use reverse channel (mobile to network) signals across multiple BTS coverage areas. By using Matched Replica correlation processing with the local and reference signals subdivided into discrete segments prior to correlation, the effects of mobile clock drift and Doppler shifts can be mitigated allowing for increased processing gain.

TECHNICAL FIELD

The subject matter described herein relates generally to methods andapparatus for locating wireless devices. More particularly, but notexclusively, the subject matter described herein relates to the use ofadvanced algorithms including matched replica and partial coherenceprocessing to detect weak signals or signals disguised by the presenceof noise allowing network-based wireless location systems increasedcapability to find the time-difference-of-arrival at multiple,geographically distributed receivers, increasing the location yield andaccuracy.

BACKGROUND

Early work relating to network-based Wireless Location Systems isdescribed in U.S. Pat. No. 5,327,144, Jul. 5, 1994, “Cellular TelephoneLocation System,” which discloses a system for locating cellulartelephones using time difference of arrival (TDOA) techniques. Furtherenhancements of the system disclosed in the '144 patent are disclosed inU.S. Pat. No. 5,608,410, Mar. 4, 1997, “System for Locating a Source ofBursty Transmissions.” Both of these patents are assigned toTruePosition, Inc., the assignee of the present invention. TruePositionhas continued to develop significant enhancements to the originalinventive concepts. Matched-replica processing is also described in U.S.Pat. No. 6,047,192, Apr. 4, 2000, “Robust, Efficient LocalizationSystem”.

Over the past few years, the cellular industry has increased the numberof air interface protocols available for use by wireless telephones,increased the number of frequency bands in which wireless or mobiletelephones may operate, and expanded the number of terms that refer orrelate to mobile telephones to include “personal communicationsservices,” “wireless,” and others. The air interface protocols now usedin the wireless industry include AMPS, N-AMPS, TDMA, CDMA, GSM, TACS,ESMR, GPRS, EDGE, UMTS WCDMA, WiMAX, LTE/SAE/eUTRAN and others.

As radio power levels decrease with increasingly strict power controlschemes and with the introduction of advanced spread spectrum codingschemes (CDMA, W-CDMA, OFDM, SC-CDMA, etc) that require continuousefficient power control, the ability of a wireless location system todetect radio signals at neighboring and geographically proximatereceivers is reduced. Location techniques used by the wireless locationsystem can include: Time-difference-of-arrival (TDOA), Angle-of-Arrival(AoA), hybrid TDOA/AoA and hybrid terrestrial TDOA with GlobalNavigation Satellite System (GNSS) measurements. A current example of aGNSS system is the United States NavStar Global Positioning System(GPS).

SUMMARY

Matched Replica correlation processing over a longer time period allowsfor radio signal detection at lower signal-to-noise ratios (SNRs). Assuggested by the Cramer-Rao bound theorem, longer integration lengthscan be used to increase the accuracy of time difference of arrival(TDOA) and Angle of Arrival (AoA) based wireless location systems

However, coherence over the entire integration length cannot normally beassumed due to mobile oscillator drift and Doppler shifts. Without theprocessing gain generated by coherent processing, detection of weaksignals in the midst of noise is more difficult.

The system described herein uses parallel processing in the correlationprocessing stage to maximize the coherence for any matched replica. Each(2-to-n) path of the correlator creates a separate time and frequencysearch space for each segment (1 -to m) of the integration length with asingle segment used for the fully coherent estimate and (m) segments inthe non-coherent estimate.

Since coherence will be possible over a population of segments and theresult of the segments can be summed, the result is a larger processinggain and thus higher correlation with the reference signal.

Additional aspects of illustrative embodiments of the present inventionare described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary as well as the following detailed description isbetter understood when read in conjunction with the appended drawings.For the purpose of illustrating the invention, there is shown in thedrawings exemplary constructions of the invention; however, theinvention is not limited to the specific methods and instrumentalitiesdisclosed. In the drawings:

FIG. 1 a schematically depicts a Wireless Location System for use with aUMTS (Wideband CDMA radio) based wireless communications system.

FIG. 1 b schematically depicts a Wireless Location System for use with aCDMA based wireless communications system.

FIG. 1 c schematically depicts a Wireless Location System for use with aCDMA all-IP based wireless communications system.

FIG. 2 a depicts a representative time-frequency-correlation map of areceived spread spectrum signal with multipath components.

FIG. 2 b depicts a representative time-frequency-correlation map withterminology.

FIG. 3 depicts the frequency shifts due to mobile velocity, mobile clockdrift and multi-path reception of the mobile's uplink signal.

FIG. 4 a depicts the reference and local signal envelopes versus timeacross a single frequency offset.

FIG. 4 b depicts the correlation of the reference and local signalsversus time across a single frequency offset.

FIG. 4 c shows the segmenting of the locally received signal andreference signals into successive, discrete subdivisions versus time ata single frequency offset.

FIG. 4 d shows the correlation of subdivision of the reference and localsignals versus time across a single frequency offset into multiplecorrelation maps.

FIG. 5 shows the operational stages of the matched replica process priorto variable coherence processing.

FIGS. 6 and 7 show different views of the operations of the variablecoherence processing approach described herein.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

We will now describe illustrative embodiments of the present invention.First, we provide a detailed overview of the problem and then a moredetailed description of our solutions.

DETAILED DESCRIPTION OF IMPROVED SIGNAL DETECTION IN A WLS

The uplink time difference of arrival (U-TDOA) location method, at itsmost basic level, relies on the assumption that a direct line of sight(LOS) path with sufficient signal energy exists between the transmitterand the receiver stations. An unobstructed LOS path does not necessarilyneed to exist between the transmitter and the receiver; however, it isassumed that the signals do not undergo a change in direction due toreflections, diffraction, ducting, etc. This assumption is made in orderto convert the time difference of arrival into the spatial straight-linedistance from the mobile station to the primary and cooperatorreceivers. These receivers, called Location Measurement Units (LMUs) bythe 3GPP, are geographically distributed. In an overlay-deployment, LMUsare typically co-located with the wireless network provider's basestations (BS) (or base transceiver stations, BTS, where these are usedinterchangeably) so that the BS and LMU may share environmentallycontrolled space, power, and antenna access. In an integrateddeployment, the LMU is incorporated into the Access Point (AP), Basestation (BS), Node B, or Base Transceiver Station (BTS) circuitry.

In reserved wireless communications bands, which currently include the700/850/1900 MHz (North American) and the 900/1800/2100 MHz (European)Cellular, GSM, PCS, DCS, AWS and UMTS frequency bands, aggressive powercontrol to increase the capacity of the wireless network and increasinguse of spread spectrum radio signaling (such as CDMA, W-CDMA, OFDM, andTD-SCDMA) serve to lower the power available to the geographicallydistributed network of receivers

The present invention also functions in unlicensed or shared bands inareas served by the geographically distributed network of softwaredefined wideband (or banked narrowband) receivers.

As originally disclosed in U.S. Pat. No. 5,327,144, “Cellular TelephoneLocation System”, Stilp et al, the signal of the emitter to be located,i.e. the mobile device, is collected by a cluster of geographicallydispersed specialized receivers (Location Measurement Units or LMUs,formerly called Signal Collection Systems or SCS's). The WirelessLocation system (WLS), when triggered by a radio monitoring sensor, alink-monitoring system, or by a request from the wireless operator,first performs radio signal metrics collection and determination of bestLMU and a set of cooperating LMUs. Each LMU preferably uses a widebandreceiver to collect and digitize RF for a sample period. The best (asdetermined by signal strength and/or quality) LMU is normally associatedwith the serving cell antenna whereas the cooperating LMUs are nominallythose in close geographic proximity to the best/serving LMU withacceptable SNR and/or Eb/No and that do not create largegeometric-dilution-of-precision (GDOP). Implementation of the LMU as abank of narrowband receivers (narrowband in the sense that theindividual receiver's bandwidth approximates a single channel) is alsopossible as detailed in U.S. Pat. No. 6,184,829 “Calibration forWireless Location System”. While the examples given for the presentinvention relate to a distributed ‘station-based’ approach where thereference signal is distributed to other candidate LMUs and the signalprocessing is performed at the receiving LMU, sufficient, low latency,bandwidth between the LMU(s) and a central processing site of computerswould enable signal processing to be performed centrally.

Digital signal processing software within the LMU models a radioreceiver and demodulates the signal of interest conventionally. Thisdemodulated ‘perfect’ reference signal is sent from the reference LMU toa group of cooperator LMUs selected on the basis of the radio metriccollection. Each cooperator LMU re-modulates the reference and uses thisin a correlation process to determine the time of arrival (TOA) of thesignal of interest at the best/serving LMU.

In a preferred implementation, copies of the re-modulated idealreference and the locally recorded received signal are subdivided intime and then the corresponding (in time) subdivisions are correlated,effectively creating NPath processing paths (where NPath is an integergreater than or equal to 1). For each processing path, the correlationof each individual subdivision can be expressed mathematically via thefollowing:

Rs=Correlation function per subdivision “s”

${{Rs}\left( {r,d} \right)} = {\sum\limits_{q = 1}^{p}{{g(r)}^{- {jwd}}*{l\left( {r - q} \right)}}}$

-   Where:-   g=reference signal-   l=Local signal-   N=number of samples in sample collection duration-   M=Number of subdivisions-   p=N/M (number of samples in each subdivision)-   s=segment index (goes from 1 to M)-   r=range-   d=Doppler and drift    Once the correlation for each subdivision(s) is performed, then the    magnitude of the correlation for each subdivision is determined and    the sum of correlations for each processing path (NPath) is    calculated. Mathematically, this operation can be shown as:

${R\left( {r,d} \right)} = {\sum\limits_{s = 1}^{M}{{Rs}\left( {r,d} \right)}}$

Ideal Fully Coherent Case

For example, FIG. 3 shows a signal data segment with a length of 1000samples over the collection duration. If the mobile was stationary (andthus no Doppler shift) and experienced no internal clock drift, thenperforming a coherent correlation provides a processing gain of 10log(1000)=30 dB. This 30 dB gain is relative to that of a single sample.

However, this gain can only be achieved if the correlation of signalsreally is coherent over the full 1000 samples. If it is not, the signalcorrelation simply falls apart and can provide zero, or even negativeprocessing gain.

A Non-Coherent Case

Use of the variable coherence technique can decrease the matched replicaprocessing susceptibility to non-coherence caused by frequency driftsbetween the reference signal and the collected signal. The processinggain from coherent correlation is 10 log(K), where K is the ratio bywhich you are increasing the data length. A common approach is to letthe reference be a single sample, then K is simply the number of samplesover which you are performing the correlation.

Variable coherent correlation involves segmenting copies of thereference and local signals, the doing complex multiplies between thereference and local signals and then summing the produced correlationproducts with a complex summation. The term variable is used since thenumber of subdivisions and/or parallel processing paths (NPath) can beadjusted depending on test results for the network topology used and thegeographic area covered. The use of the variable coherence technique canalso be toggled depending on sample collection duration, with shorterdurations using only the full coherence case.

As an example, the collected signal is segmented into discrete,successive subdivisions. Using the same reference signal and collectedsignal duration, each with 1000 total samples over the same time period,the correlated signal can be broken up into 10 consecutive, discretesubdivisions of 100 samples each.

Assuming coherence over each subdivision duration (each of the 100sample segments), the processing gain is 10 log(100)=20 dB.

The 10 segments can then be non-coherently summed (or take the magnitudesquare of each segment and then add the magnitudes together, rather thancomplex summation). The processing gain from the non-coherent summationis 10 log(sqrt(P)), where P is the number of segments that are summed upnon-coherently. So, for this example of 10 segments of a 100 sampleseach, assuming coherence over each subdivision 406, the non-coherentprocessing gain is 10 log(sqrt(10))=5 dB. The total variable coherenceoperation processing gain=25 dB=20 dB (from coherent correlation of 100samples)+5 dB (from the non-coherent sum of the 10 segments).

By subdividing the signal up into 10 segments we only get 25 dB ofprocessing gain compared to the 30 dB we would get for a fully coherentcorrelation. However, if the signal is not coherent, which is likely forlong sample periods when the mobile may move (resulting in Dopplershifts) or the mobile timing reference drifts, the fully coherent casefalls apart, and the variable coherence approach still yields 25 dB ofprocessing gain.

However, since the coherence gain cannot be known a priori, multipleprocessing paths, each with a different number of processing paths(these paths may be parallel or serial dependent on the signalprocessing power and configuration available), are created for eachlocation estimate.

The variable coherence technique can be recursive. If sufficient time isallowed by the quality of service parameters, the entry into variablecoherence operation can use the result of a first variable coherenceprocessing run to feed a second cycle. If, for instance, the correlationamplitude of correlation processing path with ‘Ma’ subdivisions issubstantially higher than all other processing paths in a cycle, then asubsequent cycle with multiple processing paths with a distribution ofsubdivisions centered around ‘Ma’ such as ‘Ma+±x” can be used in anattempt to optimize the total processing gain.

FIG. 1

Overlay WLS Environments

FIGS. 1 a, 1 b, and 1 c are illustrative of the types of wirelesscommunications networks that the present invention functions within.While the following subsections describe exemplary implementations ofthe communications system as a UMTS, IS-95 and CDMA2000 cellularcommunication systems, the teachings of the present invention areanalogously also applicable to other wideband, spread spectrum packetradio communication systems that are implemented in other manners.

FIG. 1 a

FIG. 1 a shows the architecture of an illustrative UMTS networkreference model for the present invention.

UE (100)

The UMTS UE (User Equipment) 100 is the logical combination of the ME(Mobile Equipment) 101 and SIM/USIM (Subscriber Identity Module/UMTSSubscriber Identity Module) 102. The UE is the formal name for the UMTShandset or mobile.

ME (101)

The Mobile Equipment (ME) 101 is the hardware element of a mobilestation and comprises of keyboard, screen, radio, circuit boards andprocessors. The ME processors support both communications signalprocessing and processing of various UE-based services that may includea UE-based LCS Client application.

USIM (102)

The USIM (UMTS Subscriber Identity Module) 102, also referred to as aSIM card, is a programmable memory device what holds the usersubscription information to the UMTS mobile network. The USIM containsrelevant information that enables access onto the subscribed operator'snetwork and to UE-based services that may include a UE-based LCS Clientapplication.

Node B (105)

The Node B 105 is the function within the UMTS network that provides thephysical radio link between the UE 100 (User Equipment) and theland-side network. Along with the transmission and reception of dataacross the radio interface the Node B also applies the codes that arenecessary to describe channels in a W-CDMA system. The Node B suppliestiming information to UEs 100 over the Uu 105 interface. The Node Baccess the Uu interface via wired antenna feeds 104.

The UTRAN (UMTS Terrestrial Radio Access Network) comprises one or moreRNS (Radio Network Subsystem). Each RNS comprises one or more RNC 107and their supported Node B's 105. Each RNS control the allocation andthe release of specific radio resources to establish a connectionbetween a UE 100 and the UTRAN. A RNS is responsible for the resourcesand transmission/reception in a group of cells.

S-RNC (107)

When a RNC 107 (Radio Network Controller) has a logical RRC (RadioResource Control) connection with a UE (User Equipment) via the Node B105, it is known as the S-RNC 107 for that UE 100. The S-RNC 107 isresponsible for the user's mobility within the UTRAN network and is alsothe point of connection towards the CN (Core Network) 112. The S-RNC 107connects to the Node B via the 3GPP standardized Iub interface 106.

D-RNC (108)

When a UE 100 (User Equipment) in the connected state is handed onto acell associated with a different RNC it is said to have drifted. The RRC(Radio Resource Control) connection however still terminates with theS-RNC 107. In effect the D-RNC 108 acts as a switch, routing informationbetween the S-RNC 107 and the UE 100.

C-RNC

The Controlling Radio Network Controller is the RNC (Radio NetworkController) responsible for the configuration of a Node B. A UE (UserEquipment) accessing the system will send an access to a Node B, whichin turn will forward this message onto its CRNC. The C-RNC is nominallythe S-RNC.

Core Network (112)

The Core Network 112 provides the functions of mobility management,exchange services for call connection control signaling between the userequipment (UE) and external networks, and interworking functions betweenthe UTRAN radio access network and external packet and switched circuitnetworks. The Core Network also provides billing functionality, securityand access control management with external networks.

LMU (114)

The Location Measurement Unit (LMU) makes radio measurements to supportpositioning of UE. The LMU may be an overlay addition to the UMTSnetwork or may be integrated into the hardware and software of the NodeB. In a UMTS wireless communications network, the LMU receives theW-CDMA based Uu radio interface for development of TDOA and/or TDOA/AoAcalculated location and velocity estimates. The LMU connects to cellsite antenna or to the Node B via a radio coupler to the antenna feed113.

Examples of a U-TDOA and U-TDOA/AOA LMU have been previously describedin U.S. Pat. No. 6,184,829, Calibration for a Wireless Location System;U.S. Pat. No. 6,266,013, Architecture for a Signal Collection System ina Wireless Location System; and U.S. Pat. No. 6,108,555, Enhanced TimeDifference Localization System, all owned by TruePosition andincorporated herein by reference.

SMLC (116)

The SMLC 116 is a logical functional entity implemented either aseparate network element (or distributed cluster of elements) orintegrated functionality in the RNC 107. The SMLC 116 contains thefunctionality required to support Location Based Services. The SMLC 113is the logical entity that provides the bridge between the wirelessnetwork and the location network (LMU 114, SMLC 116, and GMLC 119) bypossessing data concerning the geographical area as well as the radionetwork topology. The SMLC 116 manages the overall co-ordination andscheduling of LMU 114 resources required for the location of a mobile.It also calculates the final location, velocity, and altitude estimatesand estimates the achieved accuracy for each. In the present invention,the SMLC 116 controls and interconnects a set of LMUs via packet dataconnections 115 for the purpose of obtaining radio interfacemeasurements to locate or help locate UE 100 in the geographical areathat its LMUs serve. The SMLC 116 contains U-TDOA, AoA and multipathmitigation algorithms for computing location, confidence interval,speed, altitude, and direction of travel. The SMLC 116 can alsodetermine which wireless phones to locate based upon triggering from theLink Monitoring System (LMS) 124 or requests from the 3GPP standardizedIupc interface 117 to an infrastructure vendor's Radio NetworkController (RNC) Station Controller 107.

GMLC (119)

The Gateway Mobile Location Center (GMLC) 119 is defined by 3GPPstandards as the clearinghouse for location records in a GSM/GPRS/UMTSnetwork. The GMLC 119 serves as a buffer between the tightly controlledSS7 network (the GSM-MAP and CAP networks) and the unsecure packet datanetworks such as the Internet. Authentication, access control,accounting, and authorization functions for location-based services arecommonly resident on or controlled by the GMLC 119. A Gateway MobileLocation Center (GMLC) is a server that contains the functionalityrequired to support LBS services as well the interworking, accesscontrol, authentication, subscriber profiles, security, administration,and accounting/billing functions. The GMLC also has the ability toaccess the GSM-MAP and CAP networks to discover subscriber identity,request and receive routing information, obtain low-accuracy UElocation, and to exert call control based on UE location. In any UMTSnetwork, there may be multiple GMLCs.

Network LCS Client (122)

A Network LCS Client 112 is the logical functional entity that makes arequest to the PLMN LCS server for the location information of one ormore than one target UEs. In the UTMS network depicted in FIG. 1, theLCS server is implemented as software and data on the GMLC 119 platform.This inclusion of the LCS server with the GMLC 119 is typical fordeployed systems. An LCS server comprises a number of location servicecomponents and bearers needed to serve the LCS clients. The LCS servershall provide a platform which will enable the support of location basedservices in parallel to other telecommunication services such as speech,data, messaging, other teleservices, user applications and supplementaryservices. The Network LCS client uses the Le interface 121 to access theGMLC. The network LCS client can communicate with the GMLC-based LCSserver 119 to request the immediate, periodic or deferred locationinformation for one or more target UEs within a specified set oflocation-related quality of service parameters if allowed by thesecurity and privacy protections provided by the GMLC-based LCS server119

Mobile LCS Client

The Mobile LCS Client is a software application residing in the ME 101of the UE 100 using the USIM 102 for non-volatile or portable datastorage. The mobile LCS Client may obtain location information via theGMLC 119 using the Le Interface 121 over a wireless data connection.

LMS

The LMS 133 provides passive monitoring of UMTS network interfaces suchas the Iub, Iur, Iu-CS and Iu-PS by means of passive probes (notpictured) reporting to a central server or server cluster. By monitoringthese interfaces, the LMS 133 may develop tasking and triggeringinformation allowing the SMLC 116 to provide autonomous, low-latencylocation estimates for pre-provisioned LBS applications. LMS 133developed triggering and tasking information is delivered to the SMLC116 via a generic data connection 123, normally TCP/IP based. The LMS133 is a modification to the Abis Monitoring System (AMS) described inU.S. Pat. No. 6,782,264, “Monitoring of Call Information in a WirelessLocation System” and later expanded in U.S. patent application Ser. No.11/150414, “Advanced Triggers for Location Based Service Applications ina Wireless Location System,” both incorporated herein by reference. TheLMS 133 functionality may be incorporated as software into the Node B105 or RNC 107, 108 nodes of the UMTS system or deployed as an overlaynetwork of passive probes.

Interfaces

The Uu interface 103 is the UMTS Air Interface as defined by 3GPP. Thisradio interface between the UTRAN (UMTS Terrestrial Radio AccessNetwork) and the UE (User Equipment) utilizes W-CDMA and eitherFrequency Division Duplexing (FDD) or Time Division Duplexing (TDD). TheUMTS radio interface is well described in 3GPP technical specifications25.201 and 45.20 1, both entitled; “Physical layer on the radio path;General description”. Specifics of the Uu radio interface as implementedin an FDD W-CDMA radio system are described in 3GPP TechnicalSpecification 25.213, “Spreading and modulation (FDD)”. Details anddescriptions of the physical and logical channels used in a FDD W-CDMAUMTS are located in 3GPP Technical Specification 25.211, “Physicalchannels and mapping of transport channels onto physical channels(FDD)”.

The Iub interface 106 is located in a UMTS radio network and is foundbetween the RNC (Radio Network Controller) 107 and the NodeB 105. TheIub interface is as defined in 3GPP TS 25.430, “UTRAN Iub Interface:general aspects and principles”.

The Iur 109 interconnects the UMTS Server or core RNC 70 with the DriftRNC 108 in the UMTS network. The Iur interface is standardized in 3GPPTechnical Specification 25.420, “UTRAN Iur Interface: General Aspectsand Principles”

The Iu-CS (Circuit Switched) interface 110 connects the UMTS RNC 107with the circuit switched communications oriented portion of the CoreNetwork 112.

The Iu-PS (Packet Switched) interface 111 connects the UMTS RNC 107 withthe packet switched communications oriented portion of the Core Network112.

The Iupc 117 interconnects the UMTS RNC 70 with the SMLC (also calledthe SAS) in the UMTS network for location estimation generation. TheIupc interface is introduced in 3GPP Technical Specification 25.450,“UTRAN Iupc interface general aspects and principles”.

The E5+ interface 118 is a modification of the E5 interface defined inthe Joint ANSI/ETSI Standard 036 for North American E9-1-1. The E5+interface 118 connects the SMLC 116 and GMLC 119 nodes directly,allowing for push operations when LMS 114 triggers are used by thewireless location system with either network acquired information(cell-ID, NMR, TA, etc) or via TDOA and/or AoA (angle of arrival)performed by the LMU's 114 specialized receivers.

The Le interface 121 is an IP-based XML interface originally developedby the Location Interoperability Forum (LIF) and then later standardizedby the 3rd Generation Partnership Program (3GPP) for GSM (GERAN) andUMTS (UTRAN). The Location-based services (LBS) client 122 is also knownas a LCS (Location Services). The LBS and LCS services resident on theLCS Client 122 are software applications, data stores, and servicesuniquely enabled to use the location of a mobile device.

FIG. 1 b

FIG. 1 b schematically depicts a representative configuration of themajor components of a wireless communications system based on thatdescribed in the ANSI/ETSI Joint Standard “J-STD-036”, Enhanced Wireless9-1-1 Phase 2. For the present invention, FIG. 2 b is used to representan implementation present invention within a TIA-EIA-95 (IS-95) basedCDMA wireless communications system with standardized nodes andinterfaces. Although originally created in support of emergency services(E911, E112), this functional network can also be used for commerciallocation services delivery in a mixed circuit switched, packet switchednetwork where the MSC 135 and MPC 141 communicate with the ANSI-41protocol using the link E3 140. The present invention resides within thePositioning Determining Equipment 143 node of the reference network.

MS

The CDMA Mobile Station (MS) 130 is a hardware software system allowinguser access to the CDMA radio interface 132 and thus the completewireless communications network and services.

The MS 130 may have a location based software application, the LBSClient 131 in residence. The MS-based LBS client uses the resourcesprovided by the MS 130 to function.

The IS-95 Base Station comprises a BSC (Base Station Controller) and oneor more BTS (Base Transceiver Station(s)). The BS 133 provides thefunctionality that enables a mobile to access network interfaces andservices over the IS-95 CDMA air interface.

The BS 133 interfaces the CDMA radio interface 132 with land-basedwireless communications system network. The BS 133 provides channelallocation to the MS 130, power control, frequency administration, andhandover (soft, softer and hard) between other proximate BS.

The A interface 134, nominally an IS-634 compliant interface for IS-95CDMA systems, interfaces the BS 133 to the MSC 135, carrying controlmessaging between the MSC 135 and BS 133 and DTAP (Direct TransferApplication Part) messaging from the MSC 135 intended for the MS 130.

The MSC (Mobile Switching Center) 135 provides the functions of mobilitymanagement, exchange services for call connection control signalingbetween the MS 130 and external switched circuit networks 147, andinterworking functions between the CDMA radio access network andexternal packet switched networks. The MSC 135 also provides callrouting and billing functionality. In some vendor implementations, theMSC 135 also provides interworking, routing, and transcoding servicesfor digital packet communications.

The MSC 135 may connect with other MSC 137 using the ANSI-41 defined Einterface 136.

The MSC 135 connects to switched circuit networks 139 with controlinterfaces such as the ISDN User Part (ISUP) as standardized (TelcordiaGR-154 and T1.113) as the Ai/Di 138 interfaces and trunks.

The J-STD-036 standardized E3 140 interface is used to connect the MSC135 to the MPC 141. E3 is an ANSI-41 based interface that includesWireless Intelligent Networking (WIN) capabilities for location.

The MPC (Mobile Position Center) 141 is the gateway between the mobilenetwork, location networks, and network-based location applications. TheMPC 141 acts as router and protocol converter between the E5 interface142 specific TCAP over TCP/IP-based, J-STD-036 defined, LocationServices Protocol, the E3 interface 140 ANSI-41 messaging and the TCP/IPbased data link 151 to external LBS clients 148. The MPC may selectamong deployed PDE 143 based on quality of service parameters includedin the E3 140 messaging.

The MPC connects to Position Determining Entities (PDE) 143 via theaforementioned E5 interface. In the present invention, the PDE 143comprises a cluster of centralized processors, the serving MobileLocation Center (SMLC) 116 and a geographically distributed populationof Location Measurement Units (LMU) 114 interconnected by a proprietaryTCP/IP-based interface 115. The LMU 114 connects to the BSC 133 viaeither a radio frequency antenna feed 149 from the BS's 133 receiveantennae or alternately a data link carrying a digitized representationof the received signal from each receive antennae of the BS 133.

Although not part of the J-STD-036 defined LBS network, the SMLC 116 maycommunicate directly with the Network LBS Client 148 and via dataconnection to the MS based LBS client 133 over a packet data connectionlink 150 to a generic Packet Data Network 147.

FIG. 1 c

FIG. 1C schematically depicts a representative configuration of themajor components of a wireless communications system and wirelesslocation system based a packet-based transport network. In this figure,the wireless communications system is assumed to be based on the IS-2000CDMA or CDMA200® system.

This packet-based (also known as the all-IP based) LBS network isdescribed by 3GPP2 standards; TIA-1020, IP based location services(3GPP2x.P0024); TIA-881, LS Authentication/Privacy/Security Enhancements(3GPP2 X.P0002); TIA-843, Wireless Intelligent Network LBS Phase III(3GPP2 X.P0009); and TIA-801, Position Determination Service forcdma2000®. The present invention would be implemented in the local PDE.

The all-IP wireless communication system depicted in FIG. 1 c includes ahome network 175 part and a visited network 176 part. In many cases theVisited Network 176 will be the Home network 175. The Home network 175and Visited Network 176 are connected together by way of a packet datanetwork 174 such as the public Internet. Each network part, Home 175 andVisited 176 comprises multiple functional entities interconnected bylocal Wireless Network Operator IP Networks 173, 180.

For the enabling of location based services, A Home Positioning Server(H-PS or just PS) 171 interconnects via packet-based connections withthe administration node 170 which supplies subscription and user profilestorage, LBS services administration and access control. For thedelivery of LBS services the H-PS 171 may interconnect to a home network175 based Network LBS Client 172, an external LBS client 177, a VisitedNetwork 176 based LBS Client 178 or an MS-based LBS Client 188. For theobtaining of current or historical location of the MS 187, the H-PS 171may interconnect via packet-based data connections to the local PDE 183.

The H-PS 171 plays the same role as a Home network MPC in IS-41 networkin respect to the roles of authentication, access control,administration, and accounting functions.

The Packet Data Serving Node (PDSN) 181 acts as the connection pointbetween the radio access and Visited Network 176. This component isresponsible for managing PPP sessions between the mobile provider's coreIP network and the mobile station

The S-PS or Serving Positioning Server 176 is a PS in a visited network.The Serving PS 176 provides position information of visiting MS torequesting entities such as the Home PS 171, Network LBS clients. Itplays the same role as Serving MPC in IS-41 network and acts as thelocal proxy for the H-PS 171 in respect to the roles of authentication,access control, administration, and accounting functions.

The BSC/PCF 182 is the base station controller/packet control functionalnode. The BSC/PCF 182 node manages interconnections and communicationsbetween the radio network 186 and the PDSN. The BSC/PCF 182 isresponsible for the transparent exchange of traffic and signalingmessages between the MS 187 and network-based destinations.

The radio network 186 comprises the actual CDMA2000 ® air interface andthe radio transmission facilities alternately called BS (base stations),BTS (Base station Transceiver Sites, AP (Access Points) and cells. Theradio network 186 interconnects the BSC/PCF 182 with the MS 187 forpacket data and packetized voice communications.

In the present invention, the local PDE 183 includes a server clusterbased SMLC 116 and a geographically distributed population of LMU 114.

The PDE 183 interacts with the MS 187 (possibly using the PS's 171, 179as proxies) to provide location services to the user via the MS-basedLBS client 188 or to other LBS Clients 172, 177, 178 based on themobile's location.

Other elements of the all-IP, packet architecture of the wirelesscommunication system for reasons of simplicity, are not shown.

FIGS. 2 a and 2 b

FIG. 2 a depicts a noise and multi-path corrupted radio communicationssignal. Frequency shifts due to velocity changes during transmission(Doppler) and reference clock drift cannot be determined a priori.

FIG. 2 b depicts the time and frequency search space used to determine amaximum correlation with of the received and the reconstructed andre-modulated reference (replica) signal.

FIG. 3

FIG. 3 is shown to illustrate the difficulty of applying the matchedreplica correlation between a reconstructed reference signal and alocally collected signal corrupted by mobile frequency reference driftand Doppler shift. As shown in the time 301 versus frequency 302 plot,the mobile channel has an assigned center frequency (fc) 303, but due tomobile frequency reference drift and Doppler shifts caused by motion ofthe mobile device during the collection duration 305, the actualfrequency of the collected signal drifts 304. This frequency driftcauses a mis-match between the reference signal and the collectedsignal.

FIGS. 4 a, 4 b, 4 c and 4 d

FIG. 4 a shows a single time-slice 205 of the local and reference signalamplitudes over the sample collection duration.

FIG. 4 b shows a single time-slice 205 of the correlated local andreference signals over the entire sample collection duration 406.Correlation of the local and reference signals over multiple frequencyand time offsets are used to produce the frequency 402, time 403, andcorrelation 401 search space.

FIG. 4 c shows a single time-slice of the local and reference signalsover the sample collection duration. In FIG. 4 c, the local andreference signals have been segmented into successive, discretesubdivisions (409 and 410) spanning the entire sample collectionduration 406.

FIG. 4 d shows a single frequency time slice of the multiple frequency402, time 403, and correlation 401 search spaces (one per subdivision409, 410). As FIG. 4 d shows each subdivision now includes anindependent correlation signal amplitude envelope within the newlycreated frequency 402, time 403, and correlation 401 search spaces.

FIG. 5

FIG. 5 summarizes the processing steps used in matched replica,station-based processing. Examples of steps 501 to 507 are illustratedin U.S. Pat. No. 5,327,144, “Cellular Telephone Location System”; U.S.Pat. No. 5,608,410, “System for locating a source of burstytransmissions cross reference to related applications”; U.S. Pat. No.6,047,192, “Robust Efficient Localization System”; U.S. Pat. No.6,483,460, “Baseline Selection Method for Use in a Wireless LocationSystem”; and U.S. Pat. No. 6,661,379, “Antenna Selection Method for aWireless Location Systems” all of which are incorporated by referenceherein.

In step 501, the wireless location system (WLS) is triggered to performa location. This trigger can be a message generated by the wirelesscommunications network (WCN), internally by the wireless communicationsystem or externally by a network monitoring application such as a RadioNetwork Monitor (RNM) or Link Monitoring System (LMS). The triggeringevent may be a single message, multiple exchanged messages, or series ofmessages containing the network and radio link parameters necessary forthe SMLC to task the LMU network.

In step 502, the SMLC tasks the LMU network via the provisioned datalinks, the SMLC selected LMUs collect radio signal strength and qualityinformation 503 for the LMU downselect in step 504 where only LMUs withfavorable metrics are used in subsequent steps. Step 503 may alsoencompass a phase where the SMLC analyzes the LMU metrics and servingcell and sector to determine the optimal LMU cluster to minimize theGeometric Dilution of Precision for the location.

In step 505, the LMU with the best (as determined by the metric andanalysis) radio signal is used to create a reference (also known as areplica) wherein the signal of interest is demodulated. The reference isthen forwarded to all LMUs in the selected cluster in step 506.

At step 507, the variable coherence processing is begun.

FIGS. 6 and 7

FIG. 6 is provided to illustrate the processing steps used in variablecoherence processing of a weak signal for TOA, TDOA and AoA basedlocation estimation.

Entry to variable coherence processing 507 starts with an evaluation ofthe duration of the sample collection period. If the collection periodis below the threshold where test locations start to suffer from theeffects of non-coherent signal processing, then variable coherenceprocessing will not be performed. Pre-processing of the samples mayoccur at this stage, for example that taught in U.S. Pat. No. 6,765,531;“System and Method for Interference Cancellation in a LocationCalculation for use in a Wireless Location System”. Due to Doppler shiftand mobile device timing source shifts, full coherence over the entiresample period is unlikely, as is the case where no coherence gain isavailable for any part of the sample (see FIG. 3 for an example ofmobile timing shifts over the sample period). Therefore, the number ofprocessing paths (Npaths) used can be varied from 1 to Mn, where Mn isthe number of samples within the sample collection duration 406 (FIG. 4a).

When Npath=1, then the local and reference signals are not subdividedand are correlated over the entire sample collection duration over apreset range of time and frequency offsets 602. This correlationproduces a single correlation amplitude, time and frequency search spacefor determination of the TDOA baseline between the reference and localreceivers.

Using variable coherence, the correlation processing is split into anumber of parallel (or serial if sufficient computational power isavailable) paths 601. In the FIG. 6 example, two partial coherenceprocessing paths 614 and 615 are created by dividing copies of the localand reference signals into m1 and m2 successive, discrete subdivisions.The illustrative example shown in FIG. 6 then has three processing pathscreated for the cases of a full coherence 613 path, a first partialcoherence path 614, and a second partial coherence 615 path.

By using the partial or variable coherence path(s) 614, 615, where theentire sample and reference periods are divided into successive,discrete subdivisions, coherence gain may be possible over each or anysubdivision. To increase the odds of more subdivisions yielding thebenefits of coherent gain, multiple processing paths 614, 615 each witha differing number successive, discrete subdivisions can be created.

The number of possible partial coherent paths is only constrained by thedigitization of the software defined radio of the LMU (an exemplary LMU,formerly SCS, architecture is defined in TruePosition U.S. Pat. No.6,266,013; “Architecture of a Signal Collection System for a WirelessLocation System”). When a partial coherence processing path is createdwith M=N subdivisions where the number of subdivisions of the samplingperiod (M) is equal to the sampling rate of the LMU (N), then thatprocessing path is deemed to be completely non-coherent. While Nsubdivisions is the limit of the resolution of the samples, the numberof subdivisions and partial coherence paths need not equal N; rather thepartial coherence processing scheme may be pre-set based on operationalexperience, or be dynamic based on the collection time duration sincethe likelihood of non-coherence increases with the collection timeduration.

Once a processing path with (M) subdivisions has been created, eachsubdivision is correlated 604, 607 with the corresponding (in absolutetime) reference signal subdivision multiple times over an arbitrary orconstrained range of time delays and frequency offsets (as detailed inTruePosition U.S. Pat. No. 6,876,859; “Method for estimating TDOA andFDOA in a Wireless Location Systems” which is hereby incorporated byreference) until a search space of correlation amplitude over thefrequency range and time period is produced for each subdivision.

For fully coherent or Npath=1 processing path 613 where coherence isassumed over the entire collection duration, the local signal iscorrelated repeatedly with the time and frequency shifted referencesignal 602 across potential time-offsets (range) and frequency-offsets(Doppler and drift) to create a three dimensional search space ofcorrelation amplitude, range, and Doppler/drift, as shown in FIG. 4 a.

For each partial coherence processing path 614, 615, the first operationof the processing path 604, 607 is the creation of the three dimensionalsearch space of correlation amplitude, range, and Doppler/drift (FIG. 4c) for each subdivision in each partial coherence processing path 614,615. For each partial coherence processing path, the magnitude of thecorrelation amplitude is then summed 605 608. This summation of thesubdivisions gives a processing gain of 10 log(M)^(1/2) for each of theprocessing paths.

The total possible gain for the independent correlation of subdivisionscreated in the first stage 601 of the variable coherence operation andthe summation of magnitude of individual subdivision correlations yieldsfor each processing path with Npath>1 of gain is:

Gain=10 log(1)+10 log(M)^(1/2)

For each processing path 613 614 615, the wireless location system thenattempts to determine the earliest arriving signal at the local receiverby searching 603, 606, 609 the generated correlation, time, andfrequency maps (search space maps as shown in FIG. 2 b). Examples ofcorrelation searches include those in U.S. Pat. No. 6,876,859; “Methodfor estimating TDOA and FDOA in a Wireless Location System” and U.S.patent application Ser. No. 11/953585; “Detection of Time of Arrival ofCDMA Signals in a Wireless Communications System”, both of which areowned by TruePosition.

The correlation result of the fully coherent path 613 is compared 610with the correlation result of the first 614 and second 615 partialcoherence processing path.

From the processing path showing the highest coherence, the correlationresult of the received versus replica signal shows the time delay andfrequency offset(s) experienced by the locally received signal inregards to the reference signal. These TDOA values are then used toperform a TDOA or TDOA/AoA location estimate 611 using, for example, theLSD algorithm introduced in U.S. Pat. No. 5,327,144, “Cellular TelephoneLocation System” or the hyperbolic baseline method introduced in U.S.Pat. No. 6,047,192 “Robust Efficient Localization System”.

FIG. 7 depicts another view, i.e. a flowchart, of a variable coherenceprocessing method as described above. As shown, the following steps areperformed:

Receive a transmission from a wireless device.

Generate a first digital sample set representing discrete samples of thetransmission over a collection duration.

Execute a first correlation process in which said first digital sampleset is correlated with a reference over the collection duration.

Execute a second correlation process in which a first set of samplesegments is correlated with said reference. The first set of samplesegments comprises m1 segments, each of the m1 segments comprising asubset of said first digital sample set, wherein m1 is an integergreater than 1 and wherein the execution of said second correlationprocess yields m1 outputs.

Non-coherently sum the m1 outputs of the second correlation process.

Execute a third correlation process in which a second set of samplesegments is correlated with said reference. The second set of samplesegments comprises m2 segments, each of the m2 segments comprising asubset of said first digital sample set, wherein m2 is an integergreater than m1 and wherein the execution of said third correlationprocess yields m2 outputs.

Non-coherently sum the m2 outputs of the third correlation process.

Search outputs of the first, second and third correlation processes toidentify an earliest arriving signal in each output set.

Compare the identified earliest arriving signal in the outputs of thefirst, second and third correlation processes and select one of saidearliest arriving signals to determine a time of arrival (TOA) value foruse in location processing.

Use the TOA value in location processing to determine a precisegeographic location of said wireless device

Conclusion

The true scope the present invention is not limited to the presentlypreferred embodiments disclosed herein. In many cases, the place ofimplementation (i.e., the functional element) described herein is merelya designer's preference and not a hard requirement. Accordingly, exceptas they may be expressly so limited, the scope of protection of thefollowing claims is not intended to be limited to the specificembodiments described above.

1. A method for determining a precise time of arrival (TOA) of atransmission of a wireless device, comprising: receiving a transmissionof a wireless device; generating a first digital sample set representingdiscrete samples of the transmission over a collection duration;executing a second correlation process in which a first set of samplesegments is correlated with said reference, said first set of samplesegments comprising a first plurality (m1) of segments, each of the m1segments comprising a subset of said first digital sample set, whereinm1 is an integer greater than 1 and wherein the execution of said secondcorrelation process yields m1 outputs; summing non-coherently the m1outputs of the second correlation process; executing a third correlationprocess in which a second set of sample segments is correlated with saidreference, said second set of sample segments comprising a secondplurality (m2) of segments, each of the m2 segments comprising a subsetof said first digital sample set, wherein m2 is an integer greater thanm1 and wherein the execution of said third correlation process yields m2outputs; non-coherently summing the m2 outputs of the third correlationprocess; searching outputs of the second and third correlation processesto identify an earliest arriving signal in each output set; comparingthe identified earliest arriving signal in the outputs of the second andthird correlation processes and selecting one of said earliest arrivingsignals to determine a time of arrival (TOA) value for use in locationprocessing; and using the TOA value in location processing to determinea precise geographic location of said wireless device.
 2. A method asrecited in claim 1, further comprising executing a first correlationprocess in which said first digital sample set is correlated with areference over the collection duration.
 3. A wireless location system(WLS) including a plurality of location measuring units (LMUs), whereinat least one LMU comprises: a receiver for receiving a transmission of awireless device; means for generating a first digital sample setrepresenting discrete samples of the transmission over a collectionduration; means for executing a second correlation process in which afirst set of sample segments is correlated with said reference, saidfirst set of sample segments comprising a first plurality (m1) ofsegments, each of the m1 segments comprising a subset of said firstdigital sample set, wherein m1 is an integer greater than 1 and whereinthe execution of said second correlation process yields m1 outputs;means for non-coherently summing the m1 outputs of the secondcorrelation process; means for executing a third correlation process inwhich a second set of sample segments is correlated with said reference,said second set of sample segments comprising a second plurality (m2) ofsegments, each of the m2 segments comprising a subset of said firstdigital sample set, wherein m2 is an integer greater than m1 and whereinthe execution of said third correlation process yields m2 outputs; meansfor non-coherently summing the m2 outputs of the third correlationprocess; means for searching outputs of the second and third correlationprocesses to identify an earliest arriving signal in each output set;and means for comparing the identified earliest arriving signal in theoutputs of the second and third correlation processes and selecting oneof said earliest arriving signals to determine a time of arrival (TOA)value for use in location processing.
 4. A system as recited in claim 3,further comprising means for executing a first correlation process inwhich said first digital sample set is correlated with a reference overthe collection duration.